Field of the Invention
The present invention relates to an image pickup apparatus for performing focus adjustment using an image signal acquired by an image sensor that photoelectrically converts an object image formed by an imaging optical system, and a method for controlling the same.
Description of the Related Art
In a digital still camera and video camera, automatic focusing (hereinafter referred to as AF) is conventionally performed by detecting a signal corresponding to a contrast of an object image detected by an image sensor and focusing on the object using a signal output from the image sensor. Well known examples image sensors include a charge coupled device (CCD) or a complementary metal-oxide semiconductor (CMOS).
In the above-mentioned method, the contrast of the object image needs to be sequentially detected while moving a focus lens along an optical axis over a predetermined movement range (a scanning operation).
When the object enters or leaves a focus detection area as the object moves (object changes position) or a defocused state of the object changes during the scanning operation (e.g., due movement of the camera), the object changes during an AF operation so that the object is difficult to accurately focus on.
A method for coping with a case where the object changes during the AF operation includes a method for overlaying and arranging two focus detection areas (range-finding areas), which differ in size, within an imaging range, and determining that the object has moved when an evaluation value greatly changes only in a larger focus detection area, to change a position and a size of the focus detection area. An example of this method is discussed in U.S. Pat. No. 6,229,568 (counterpart Japanese Patent Application Laid-Open No. 09-018768).
However, in a conventional technique discussed in U.S. Pat. No. 6,229,568, described above, the focus detection area (range-finding area) is changed after it is detected that the object changes during the AF operation. Therefore, the AF operation needs to be performed again so that an AF time is lengthened. When the AF time is lengthened, the reliability of an AF evaluation result is damaged if the object moves.
In the conventional technique discussed in U.S. Pat. No. 6,229,568, described above, to obtain a reliable result, focus evaluation needs to be performed for at least two focus detection areas (range-finding areas). Therefore, the AF calculation load is large.